Non-destructive and rapid materials characterization methods have greatly expanded our understanding of fundamental materials behavior, and this knowledge has found numerous applications throughout society1. For example, a material's mechanical properties, such as the Young's modulus, degrade over time and can be used as a predictive indicator or marker of failure. Therefore, by combining failure analysis with mechanical deformation diagnostic measurements, the remaining lifetime of key aircraft components such as helicopter blades can be predicted, allowing preventative maintenance to be performed2. Recently, this type of analysis has been translated to the bio-domain and applied to more visco-elastic materials3,4. These types of materials exhibit significantly different mechanical behaviors and have more complex sample handling requirements; for example, experiments with human tissue samples need to be performed in biosafety cabinets. Given these types of regulations, the conventional measurement instrumentation (a load-frame or load cell) is no longer suitable. Therefore, researchers are increasingly turning to alternative methods, such as nano-indentation, atomic force microscopy (AFM), and sonoelastography, to solve these challenges5-7. In previous work, these techniques have successfully characterized the Young's modulus of biomimetic samples and of tissue8,9. However, these methods all face unique hurdles: nanoindentation generates results which require complex analysis and it has a large footprint, AFM is extremely sensitive to environmental vibrations, and sonoelastography requires manual, uncontrolled compression for signal generation. Therefore, a new system is needed which: 1) has a small footprint suitable for biosafety cabinet operation or other point-of-caré settings, 2) maintains high sensitivity, 3) uses disposable or sterile sensors, and 4) analyzes samples non-destructively and quickly.
The most approach for meeting these requirements is to reduce the number of components and simplify the operation. One promising method is based on optical fiber sensors; in particular, optical sensors based on polarization-maintaining (PM) optical fiber10. This method meets the requirements for disposability, non-destructive, and rapid analysis. In addition, these devices have a high tolerance to environmental noise, and the theoretical sensitivity is comparable. However, despite their strengths, previous work with polarimetric stress and pressure sensors has typically required free-space optical components, such as polarizers, which require alignment and are not portable11,12. Additionally, these systems relied on an analyzer to probe the polarization state of the fiber at the output. This method reduces the amount of information that can be obtained from these types of sensors, limiting the overall utility. However, significant innovation in the system design and in the signal analysis was needed in order to realize a portable system based on this strategy.